Aircraft engine air inlet performance at low speed, in the range of Mo=0.20, ,and at high cruise speed, in the range Mo=0.70-0.80, can directly affect the aircraft size and/or the mission capability. During low speed operation inlet total pressure recovery must be maximized to achieve competitive levels of single engine rate of climb. In the high speed cruise operation, inlet spillage drag must be minimized to achieve low values of installed specific fuel consumption. If spillage drag causes cruise fuel consumption to be too high, the aircraft will have to be relatively heavy to meet a given radius requirement. If low speed single engine rate of climb cannot be met due to low inlet total pressure recovery, then the engine will have to be relatively larger, driving the aircraft to be even heavier.
Both low speed and high speed requirements can generally be met by employing a thick inlet cowl lip, having generous internal and external contours. The more generous contours preclude the laminar airflow separation that otherwise occurs in both modes of operation. But modern requirements that reflect high survivability characteristics through low observable signatures dictate that inlet cowl lips be thin.
Several approaches to providing good low speed performance for thin lip inlets have been employed in the past. These include auxiliary inlets of various designs, rotating lips, and blowing of high pressure air. Auxiliary inlets provide extra openings in the duct having generous entry contours, thus reducing the amount of inflow that will flow over the sharp lips of the main inlet. Rotating lips transform the sharp leading edge into a more generous contour.
Variable position lips, similar to the rotating lip, have been employed to preclude the external lip laminar airflow separation responsible for large spillage drag at high speeds. There is no known application of blowing high pressure air over external lip surfaces to preclude separation and thereby reduce spillage drag.